This invention relates to a polymethacrylate or polycrotonate ultrathin membrane, a method for producing the membrane and an electrical device or element provided with such a membrane.
An ultrathin polymeric membrane used heretofore in electrical or optical devices or for patterning is a smooth and homogeneous thin film having uniform molecular orientation. One of the methods for producing such an ultrathin polymeric membrane is the Langmuir-Blodgett method, referred to hereinafter as the LB method. The LB method resides in forming a dilute solution of organic molecules in an organic solvent, developing the solution on a clean water surface, and compressing a gaseous membrane remaining after vaporization of the solvent in the horizontal direction to form a solid membrane having molecules packed tightly together, and transferring and stacking the membrane in plural layers on the surface of a solid substrate. The thin membrane thus formed on the substrate is called the LB membrane, see for example the literature by K. B. Blodgett, J. Am. Chem. Soc., 55, 1007 (1935) and K. Fukuda et al, J. Colloid Interface Sci. 54,430 (1976). A feature of the LB membrane is that membranes within a range of thicknesses from an ultrathin membrane of the order of a molecule to a stacked membrane of a desired thickness, i.e. a multilayered membrane may be produced and that the membrane is smooth and homogeneous with uniform molecular orientation. Thus, the LB membranes are expected to be used for a variety of electronic materials and formation of LB membranes of straight-chain fatty acids having not less than 16 carbon atoms, or alkaline earth metal or cadmium salts thereof, has been considered extensively, see for example "Bunshi Syugotai" by K. Fukuda and H. Nakahara in "Kagaku Sousetu 40" pages 80 to 104, 1983 and literatures cited therein. However, these LB membranes of fatty acids and metal salts thereof are low in mechanical strength or thermal resistance and hence cannot be put to practical applications. Accordingly, it has been suggested to form polymerizable fatty acid into an LB membrane prior to polymerization or to polymerize on the water surface followed by formation of an LB membrane, see for example the above literatures. However, with these polymerization methods, the membrane is frequently constricted or cracked during polymerization, while it is extremely difficult to transfer the membrane onto the substrate surface.
Although it is possible to form an LB membrane of a polymer material and to transfer and stack the LB membrane on the substrate, a polymer material in general is in the state of intricately entangled strands, even in a dilute solution, so that a gaseous membrane is not formed when the polymer material is spread on the water surface, with resulting difficulties in the formation of LB membranes. The polymer chain takes the form of a rod-like structure to form an LB membrane in exceptional cases wherein polypeptide or polyitaconate is used, as reported by J. H. McAlear et al, Symposium on VLSI Technology, Digest of Tech. Paper,82 (1981), K. Shigehara et al, J. Amer. Chem. Soc., 1237, Vol. 109, (1987).
It has also been tried to provide a solution of synthetic polymer material on the substrate by spin coating for forming an insulating layer on a variety of devices. However, the properties required of the insulating layer include thermal resistance of at least 200.degree. C., thermal and chemical stability, moisture proofness and excellent mechanical and electrical properties. There are only a limited number of polymer materials which were known to have these properties, such as, for example, polyimide, polyether sulfone, polyphenylene sulfide, polysulfone, polyphenylene oxide or polyethylene terephthalate. These polymer materials are dissolved in an organic solvent to form a dilute solution which is then spin coated on a substrate prior to vaporization of the solvent to form an insulating layer. As the solvent for polyimide or polysulfone, dimethylacetoamide or N-methyl pyrrolidone is employed. However, these solvents have high boiling points and are polar solvents, so that they are low in vaporization speed and tend to remain in the insulating membrane. In addition, since the polymer solution has high viscosity, technique of higher level is required to form a smooth homogeneous membrane.
On the other hand, in the field of image display, various developments have recently been made in the display method by liquid crystals since the display with a quicker response may be made with lower power consumption and a display from a small area to a larger area is also feasible. The critical point in the preparation of the liquid crystal device is how to array the liquid crystal molecules in an orderly manner. To this end, it is critical to provide the liquid crystal substrate with proper orientation characteristics, and a variety of processing methods have been known to date for achieving such orientation characteristics.
As the processing methods for providing orientation characteristics, there are known the methods of solution coating, plasma processing, rubbing, vacuum evaporation and lift coating, as disclosed in "Recent Technology of Liquid Crystals--Physical Properties, Material and Application" by S. Matsumoto and I. Tsunoda. Most common place among the above methods is the rubbing method, in which the substrate itself is rubbed in a predetermined direction by cloth or leather, or a skin layer of an organic or inorganic material is formed on the substrate surface and rubbed for orientation processing. By such orientation, the liquid crystals are oriented in a direction parallel to the rubbing direction. This method is currently used on the production site of liquid crystal cells In general, a polyimide resin is coated on a substrate for a liquid crystal and the so-formed skin layer is rubbed to control the orientation of the liquid crystal molecules.
Currently, a higher contrast and a quicker response are required of the liquid crystal display device. To this end, it is necessary to reduce the thickness of the orientation membrane itself to the order of Angstroms. It is also necessary to control the orientation more precisely. Although a polyimide resin, a liquid crystal material known to date, is superior in thermal resistance, mechanical strength and liquid crystal orientation characteristics, it needs to be applied by spin coating, roll coating, immersion coating, spray coating or gravure coating, in order to reduce the membrane thickness to as small a value as possible to produce the orientation membrane. However, the solvent used for dissolving the polyimide resin is polar and, in addition, it has a high boiling point, so that it is difficult to prepare the thin membrane by coating.
Therefore, with the current method of coating or spin coating the polyimide resin and rubbing the coated layer to form the orientation membrane, it is difficult to reduce the membrane thickness further and to produce a homogeneous and impeccable membrane, such that orientation cannot be controlled in a desired manner by the rubbing method.
It has also been proposed to stack or deposit monomolecular solid membranes in plural layers on a liquid crystal substrate by the LB method, as disclosed in the Japanese Unexamined Patent Publication No.274451/1988. However, the low molecular polymer LB membranes cannot be used practically since they are insufficient in miscibility with liquid crystal or in chemical, thermal and mechanical resistances, whereas the polymerizable LB membrane is subject to constrictions or membranous defects due to contraction caused during polymerization.
On the other hand, electrical devices such as varistors, thyristors, diodes, photodiodes, light emitting diodes, transistors or LSIs composed of integrated circuits formed by these electrical devices, may basically be classified into MIM (metal/insulator/metal) device, MIS (metal/insulator/semiconductor) device, MS (metal/semiconductor) device or Scottkey element and SIS (semiconductor/insulator/semiconductor) device. Among these, for MIM, MIS and SIS devices which are in need of the insulating layers, referred to hereinafter as the I layers, a method has been proposed which consists in thinly oxidizing a substrate of aluminum or beryllium or a silicon surface to form a metal oxide and/or an SiO.sub.2 insulating layer and forming a counter electrode. However, this method cannot be applied to substrates of metals and/or semiconductors other than those mentioned above and, when above all the semiconductors other than Si, inclusive of the compound semiconductors, are used, the method cannot be applied to MIS type devices which may be applied extensively to, for example, diodes, photodiodes, light emitting diodes, field effect transistors or thin film transistors. Therefore, if the insulating thin membrane of an organic compound is used as a sole layer, the totality of the combinations are enabled. The insulating thin membrane used for this purpose is required to be smooth and homogeneous while being 20 to 1,000 .ANG. in thickness.
On the other hand, while the LB membrane produced by the LB method is thought to be promising as the material for electronics, as mentioned hereinabove, it is only poor in mechanical strength and thermal resistance and hence cannot be used practically. Moreover, it is subject to constriction or cracking at the time of polymerization, while it is extremely difficult to transfer the membrane onto the substrate.
In general, a soft linear polymer material is in the aggregated state presenting intricately entangled strands in any dilute solution, and cannot be formed into a gaseous membrane when evolved on water surface so that it is unfit to be formed into an LB membrane. As an exception, an LB membrane of polypeptide has been reported, as mentioned above. However, this membrane may be dissolved only in a specific multicomponent solvent, such as chloroform/trichloroacetic acid/methanol, while trichloroacetic acid, which is an indispensable ingredient for maintaining solubility, tends to deteriorate the surface of a metal used as a substrate.